It is well known that many types of electronic equipment tend to radiate electromagnetic energy when operated in the high frequency ranges. Such radiation is generally undesirable for several reasons. Information being processed by the electronic equipment, for example, might be compromised in a security sense by such unwanted radiation or leakage. Spurious radiation might interfere with or distort information being processed in nearby equipment. Federal Communications Commission (FCC) regulations on such spurious electromagnetic radiation might be violated. Finally, such radiation may be considered to be a health hazard. It is therefore common to provide shielding around such electronic equipment to avoid such unwanted radiation.
In the past, electronic equipment was typically housed in metallic cabinets which served as natural radiation barriers. More recently, however, housings for electronic equipment has tended to be plastic for economic and aesthetic purposes, but thereby aggravating the radiation leakage problem. In order to prevent unwanted radiation from electronic equipment, it is common to revert to the earlier equipment designs, completely enclosing the equipment in a metallic housing which, when connected to a common potential level, provides an electromagnetic and electrostatic shield against unwanted radiation. If the equipment includes a cathode ray tube, light emitting diodes (LEDs) or other types of display devices, the shielding enclosure must include a portion which is transparent to visible light while opaque to electromagnetic radiation in the microwave region. A finely woven conductive mesh stretched across the face of the display surface and intimately connected to the rest of the metallic enclosure has been used to provide a total shielding enclosure for such electronic equipment. Fabrication of such a mesh is shown in B. E. Kuhlman et al. U.S. Pat. No. 4,412,255, granted Oct. 25, 1983, and R. D. Paynton U.S. Pat. No. 4,514,585, granted Apr. 30, 1985. Providing such intimate electrical connection of the metallic mesh to the metallic enclosure has presented difficult and expensive fabrication problems which has tended to make such shielded enclosures considerably more expensive than the more common unshielded equipment. Some of these approaches are shown in J. Choder et al. U.S. Pat. No. 4,246,613, granted Jan. 20, 1981, W. D. Coats et al. U.S. Pat. No. 4,381,421, granted Apr. 26, 1983, L. H. M. Jandrell U.S. Pat. No. 4,468,702, granted Aug. 28, 1984, and R. R. Dickie et al. U.S. Pat. No. 4,686,576, granted Aug. 11, 1987.
A common technique for providing the metallic enclosure for electronic equipment housed in plastic cabinetry is to coat the inner surface of a plastic housing with a conductive layer by spraying, painting or electro-depositing such a layer. Providing such a metallic layer is expensive and of limited value due to the limited effectiveness of such coatings due to their limited conductivity, and the large number of orifices in such housings for circulating air for cooling and for connections to other equipment by way of cables passing through such orifices. Moreover, making a good continuous electrical connection to such a metallic coating is difficult and expensive.
As a result of these and other fabrication problems, it is common for manufacturers of such equipment to provide two or more totally different equipment enclosure designs for shielded and unshielded electronic equipment. The shielded designs tended to be considerable more bulky, expensive and less aesthetic in appearance than the comparable unshielded designs due to the severe fabrication constraints on such designs. As a result, the use of highly shielded designs for electronic equipment including display surfaces has been restricted to small segments of the market which could bear the added costs. Such market segments include the military market and the market for use in environments especially sensitive to electromagnetic radiation such as hospitals and laboratories.